Oscillograph Tube, Cathode-Ray

a cathode-ray tube for converting electrical signals into a visible graphic image. The cathode-ray oscillograph tube is the basic component of an electronic oscillograph, where it is used to observe the shape and to measure the amplitude, duration, and other parameters of electrical signals. Cathode-ray oscillograph tubes are also used for the photographic or visual recording of signals in other electronic, electrical, medical, and scientific apparatus. The first such tube was made by the German physicist K. F. Braun in 1897.

The beam of electrons, or cathode ray, is produced by an electron gun. After passing through a focusing and deflecting system, it impinges on a cathodoluminescent screen that covers the interior of the tube’s face. In response to the deflection system, the beam, which is practically inertia-free, traces a luminous line on the screen, thus graphically representing the signal under study as a function of time or some other parameter. The screen is rectangular or circular and has a diagonal length or diameter ranging from 20 to 700 mm; the ratio of the lengths of the sides of rectangular screens ranges from 1:1 to 1:2. The phosphors that are used for visual observation have a white or green glow, while those used for photographic recording of the beam have an azure or dark blue glow. The afterglow of these phosphors persists on the screen for as little as 10^–7 secs, and up to 20 secs. Frequently the inner surface of the tube’s face is provided with a parallax-free scale for measurement.

Depending on the nature of the signals to be registered and the reproduction features of the apparatus, the most important cathode-ray oscillograph apparatus are classified as having low frequency, broad band (which can be high and superhigh frequency), high voltage, electrostatic storage, multigun, or radial-beam deflection tubes. The low-frequency tubes are designed for a frequency band that ranges from zero to tens of megahertz (mHz). They generally have electrostatic focusing, an electrostatic deflection system, adequate sensitivity with a beam deflection of up to 5 mm per volt (V), and a green glow on the screen.

The broad-band tubes accommodate signals in a frequency range from zero to several gigahertz (GHz). They exceed the other types of tubes in sensitivity, with a beam deflection of up to 10 mm/V; in writing speed, which can be up to tens of thousands of km/sec; and in resolution, with a line width of 50 to 300 microns (M). The wide frequency band is achieved by using a traveling wave with an interaction circuit, which is usually a helix, instead of signal-deflection plates. The high writing speed is obtained by high-voltage acceleration of the electrons after deflection. This system is an example of post-acceleration for which from 8 to 25 kilovolts (kV) are required.

High-voltage tubes, which are used to register pulses of high voltage, have a very low sensitivity—from 0.05 to 20 mm/kV—and a high electrical resistance of up to several tens of kV.

Electrostatic storage tubes, which are used to store information in the form of electrical signals and to reproduce the signals on a screen, have a very high retention time for the recorded information—from several tens of seconds to several hours. Multigun tubes, which are used to observe several simultaneous processes on a single screen, usually contain two, five, or ten independent low-frequency systems for forming beams within a single envelope.

In radial-beam deflection tubes, which are used to study phenomena in a polar system of coordinates, the beam is swept in a circle by means of two pairs of deflection plates. The signal voltage is fed to the plates of a conical capacitor that deflects the beam radially. The specifications of these tubes are similar to those of the low-frequency type.